Health and reproduction: the sex-specific clinical profile of great tits (Parus major) in relation to breeding

To describe the changes in clinical profile of individuals that occur durin g different stages of the reproductive cycle, 17 condition indices includin g leukocyte counts, plasma protein and metabolite concentrations, hematocri t, and hemoparasite prevalence were examined in prelaying, and brood-rearin g great tits (Parus major). The metabolic profile pointed to increased fat metabolism during the prebreeding period. Protein metabolism did not indica te nutritional limitation. Prelaying birds had elevated gamma-globulin leve ls, which may indicate either,greater investment in humoral immune defence or, alternatively, greater exposure to immune challenge than for breeders. The sexes did not differ with respect to prevalence of hemoparasites, but f emales had generally lower albumin/globulin ratios than males, which might indicate a greater predisposition to infectious diseases. Breeding females had higher hematocrit values and heterophile/lymphocyte ratios than males, suggesting that brood rearing imposes a greater work load and exerts greate r stress on females. In contrast to the breeding period, males seemed to be more stressed than females prior to egg laying, as indicated by their lowe r lymphocyte counts and higher heterophile/lymphocyte ratios.